There is a known process which is based on the use of two wires connected only to two phases and of a contact electrode for application by a finger of the user. This process involves the use of a dividing bridge essentially made up of resistors and presents an inconvenience if insulating gloves are worn. Furthermore, the capacitive connection to the user causes a phase shift which gives a false reading. Moreover, it cannot be assured that the electrical reference of the system, even without gloves, is ground potential. For the safety of the user, there is no insulation but only current limiting, which is only effective under normal conditions of use.
Another known process uses two wires with storage of one phase, which is compared to another phase, upon opening of a time window. This process has the disadvantage of requiring operation in a very short time on a network and does not function correctly on generating units because of the instability. Furthermore, it only functions over a very reduced range of frequencies, and it is necessary to be able to differentiate the neutral phases in order to avoid erroneous readings of the direction of rotation due to a poor connection.